Production of lead chromate, etc.



Patented Sept. 19, 1939 a UNITED STATES PATENT OFFICE PRODUCTION OF LEAD OHROMATE, E'ro.

Omar F. Tarr and Winslow H. Hartford, Baltij more, M11, assi'gnors to Mutual Chemical Co.

of America, New York, N. Y., a corporation of New Jersey No Drawing. Application January 6, 1937, 7 Serial No. 119,260

12 Claims. (Cl. 13467) This invention relates to improvements in the been used up to form lead chromate or lead production of lead salts and particularly improvechromate-containing compounds.

merits in the manufacture of lead chromate, basic The nature of the reactions which take place lead chromate and pigments containing these appears to be illustrated, or capable of being compounds as ingredients. The invention informulated, by the following. equations, based cludes an improved method of preparing such upon the use of lead oxide or litharge as the incompounds and the improved product so prosoluble lead compound employed and the addiduced. tion of admixed sodium bichromate and sulfuric 10 The process of the present invention enables acid: 10

glogggs t 12f outstanding pur ty of tone, tinting (1) 4PM)+H2s04+Na2Cr2o7+H2O= g and fastness to light to be produced. ZPMOH) Pbc O N so, It ofiers a new procedure for controlling the anr 4+ a2 4 y color an phy i al pr per f the (2) 2Pb 0H 2.PbCr04+H2s0i+Na2Cr20v= v suiting pigment. It also'enables economical raW 4PbCrO4+Na2SO4+3H2O 15 materials to be employed. I

The improved process of the present invention W lead sodium bichimate and Sulmakes use of insoluble lead compounds as the acld a used m Fiquaitlons pllrposes starting material, for example, the oxide, hydroxlllustraitlon the mventlqn Is not hmlled to ide, basic carbonate, carbonate, etc. It has herepartmulafr .Compounds m actual plfactlce' 2.0 tofore been proposed to make use of such in- 1 apparent m thegbove, equatlons that soluble lead compounds in the manufacture of Qurmg the first half of the reactlim, or l lead chromate, etc., but the present process representid by e? equaitlon' the ongmal utilizes such insoluble lead compounds to pan basic lead compound is present in excess to conticular advantwa trol the pH of the solution in such a manner that 2.3 The process of the present invention takes adthe Solution is always alkalmm and h durmg vantage of the fact that the Smcalled insoluble the second half of the reaction, or during the relead Compounds of the general type of those action representedby the second equat on, basic tioned above possess a definite solubility in Water, lead chromate, hkewlse present 9 control which, although very small, nevertheless furnishes P Solutlon W the 1 9 15 always an amount of vlead ion suflicient for the realkahney has a PH greater man 30 quirements of the process, as hereinafter de- .W t the r u d progressive: addition of scribed Along with. the solution of a small the admixed acid and chromate or bichromate at amount of lead there is also liberated from the a rate not exceeding the rate of solution of the insoluble lead compound, by the solution proca compounfl' andin Spite of the large F ess, a certain amount of hydroxyl ion, which also of foreign amons present (sulfate anions 1n the 35 plays an important part in the process, For 1 above illustration), there can be no excess of stance, in the case of lead Oxide, the reaction may hydrogen ionto redissolve the lead chromate and be represented as follows; consequently no appreciable formation of an undesired lead salt (such as the sulfate) when the 40 PbO+H2O:Pb(OI I)2=Pb+++2(OH)- stoichiometrical.quantities of reagents indicated 40 by the equation are employed.

It is a novel and characteristic feature of the process of the invention that a pure lead chromate is prepared whenever a mixture of a bichromate and an acid is added in the stoichio- We have found that if an insoluble lead compound of the type above mentioned is rapidly agitated in a large volume of water, and a mixture of an acid anda soluble chromate or bi- "chromate is run in at a rate not exceeding the rate metrical quantities indicated by the above equa of solution of the lead compound, the lead comn to slurry'of finely divided lead compound Pound is gradually and progressively converted of the above mentioned insoluble type in water, into lead chromate-containing compounds, esregardless of th cid, employed i 'u t 'pecial1y suitable for use as pigments; and that with the bichrom'ate added. I 50 essentially all the lead compound can be so con- So also, basic lead chromates, and mixtures of fverted. By regulating the rate of addition, an albasic lead chromates with normal lead chromate kaline condition is maintained throughout the (known as chrome oranges) may be readily process up to the point where all or essentially all I prepared bythis method by the use of a deficiency of theinitial lead compound, e. g. litharge, has both of bichromate and of acid, e. g by inter- 55 50% or more of the bichromate-acid mixture has been added.

Where lead chromate, or basic lead chromates, or mixtures of basic lead chromates with normal lead chromate, are desired, these are readily produced in a state of high purity and without admixed sulfate, even though a large excess of sulfate ions is present during the process. By keeping the reaction mixture continually alkaline during the process, the lead chromate (or basic lead chromate, etc.) is formed free from sulfate so long as the reaction is kept continually alkaline and the proper proportions of chromate and acid are added. Even when the reaction is stopped at the halfway point, or at a point where only part of the litharge has been converted to chromate, there is essentially no lead sulfate present when the proportions are correct, and when the chromate and acid are properly added to maintain an alkaline reaction.

The invention includes the further discovery that mixtures of lead chromates with other lead salts can be advantageously prepared, and in particular mixtures of lead chromate and lead suifate of the types known as lemon or primrose yellow, but of improved properties. For the production of such composite lead pigments or products, an excess of acid over bichromate is employed to react with the lead compound, but the excess acid is so added that the reaction mixture is maintained alkaline throughout. When an excess of acid is thus added, it may be added separately, at the beginning of the reaction, and

the bichromate-acid mixture then added, with gradual addition in each case and while maintaining the reaction mixture alkaline; or the bichromate-acid mixture may contain the desired excess of acid; or a combination of these two methods maybe employed.

Thus, where mixtures of lead chromate and lead sulfate are to be manufactured, we have found that the percentage of lead sulfate in the finished product is substantially quantitatively equivalent to the excess of acid over bichromate employed, and that this is so even where the excess acid is some other acid than sulfuric acid, e. g., acetic acid; and that this result is obtained even though the excess acid is added at the start of the reaction and before the bichromateacid mixture is added, as well as when the excess acid is present in the added bichromate-acid mixture, as long as sulfate ion is present in an amount at least equivalent to the desired amount of lead sulfate.

It is a characteristic and important advantage of the present process that it enables cheap and readily available starting materials to be used, for example, the cheapest lead compound, litharge; the cheapest bichromate, sodium bichromate; and the cheapest acid, sulfuric acid; and that it enables these to be completely and efliciently utilized for the production of lead chromate pigments.

The invention will be further illustrated by the following specific example for the production of a medium yellow pigment, the parts being by weight:

223 parts of litharge are agitated in 6,000 parts of water. A bichromate-acid mixture is prepared containing 49 parts of sulfuric acid or its equivalent and 149 parts of sodium bichromate dihydrate or its equivalent in 500 parts of water. The litharge is advantageously in a finely divided condition to give the maximum rate of solution.

rupting the reaction described above when about Rapid agitation is maintained while the acidbichromate solution is added very slowly over a period of several hours, and this addition is so regulated as to maintain the reaction mixture alkaline throughout. If the bichromate-acid. mixture is added too rapidly, the particles of litharge will become coated with a film of lead chromate and with consequent tendency for the reaction to be incomplete and an inferior product produced; but with regulated addition at a rate not exceeding the rate of solution of the litharge, su'ch incomplete reaction and the production of inferior products can be readily avoided.

In the above example, a portion of the sulfuric acid used in the reaction mixture may be replaced by an equivalent quantity of another acid, such as acetic acid, which may, if desired, be added to the litharge at the start of the reaction. Where acetic acid is thus added, at the outset, and before the addition of the bichromateacid mixture, it will react to form lead acetate which is soluble; but when the bichromate-acid mixture is added, the lead acetate is converted into sodium acetate without regeneration of acetic acid.

When it is desired to produce a mixed chromate-sulfate pigment, the excess of acid, whether sulfuric or some other acid, may be added at the outset and before the addition of the bichromateacid mixture; or it can be added as a part of such mixture.

When sulfuric acid is added to the lithargewater suspension at the outset, it should be added gradually and While maintaining alkaline conditions throughout, so that the litharge does not become coated with the precipitated lead sulfate. By proceeding in this way, a regulated and desired amount of a lead sulfate is first produced and the chromate is then subsequently produced by the regulated addition of the bichromate-acid mixture. When the excess acid is first admixed with the bichromate and the resulting bichromate-acid mixture added, a mixed chromate-sulof lead sulfate PbSO4, the procedure may be similar to that of the above example, with the quantity of acid and lead compound remaining unchanged, but with the bichromate used reduced to 111.8 parts by weight. With these proportions of reagents there is an excess of acid over bichromate, and insufiicient chromate added for the formation of lead chromate from all of the litharge employed. In this case, as the bichromateacid mixture is gradually and progressively added, both chromate and sulfate are formed progres- .sively throughout the process.

The present process can be carried out at different temperatures since the relative solubility relations involved are not changed to any great extent by raising and lowering the temperature. Considerable variation in the color of the finished pigment may be obtained by variation of the temperature, the colors tending toward somewhat deeper tones as the temperature is raised. The use of hot solutions is ofadvantage where more rapid solution of the lead compound is desired.

In the first example given above, for the manufacture of a medium yellow pigment, the acid employed is sulfuric acid and theproportions of acid and bichromate are such as to give a substantially pure chromate pigment, without admixture of any material amount of sulfate, even though a this example, other acids can be used and the same or a similar pigment obtained.

In-the example given-abovefor the manufacture of a lemon yellow, which is a mixed chromate-sulfate pigment, the acid employed is also sulfuric acid; but other acids can also be employed provided sufiilcient sulfate ion is present to produce the'desired lead sulfate, and provided proper proportions of reagents are employed.

In the manufacture of medium yellow pigments ;it is sometimesdesirable to incorporate small quantities of lead sulfate in the pigment, and such amounts are conveniently obtained by adding the desired quantity of an acid to the batch before adding the bichromate-acid mixture, and

.with a corresponding reduction in the amount of such mixture added. Where sulfuric acid is the acid thus added at the outset, lead sulfate will be first formed from part of the litharge, and the chromate will be subsequently formed from the :remainder of the litharge when the bichromatewhich case, as above pointed out, chromate and sulfate are both formed during the process.

The present invention is of moreor less general application to the production of mixed chromate pigments containing the chromate admixed with other insoluble lead salts, such as the molybdate, tungstate, etc., by the proper choice of reactants, e. g., by adding a soluble molybdate or tungstate to the bichromate-acid mixture, and withcorresponding reduction in the amount of bichromate employed. Similarly mixed pigments containing more than two admixed lead salts can be prepared, such as chromate, sulfate and molybdate, or chromate, molybdate and tungstate, etc. Alumina may be incorporated in the product by replacing someof the acid by the desired equivalent of aluminum salt.

It is characteristic of the present process that it gives voluminous precipitates. The volume of the precipitates obtained by the present proc- '1 ess is much greater than that of those prepared by any process with which we are familiar. For example, 1 pound of the chromate pigment as precipitate may occupy as much as six gallons of space when settled from water. This material is readily washed by decantation, but the exare also instrumental in decreasing somewhat the volume of the precipitate. By these means the physical properties and the color tone and strengthof the product may be controlled while its chemical analysis may be controlled in the 6; manner described above. g M

In physical properties the products of the pres-- ent invention are distinctly different from lead chromate-containing compounds prepared by any known method with which we are familiar. The particle size is very small, and therefore the pigments prepared by the present process possess extraordinary fiocculence when dispersed in water, high tinting strength, high oil absorption, and a yellower mass tone than those prepared in the usual fashion.

We are aware that it has been proposed to prepare lead chromate pigments by causing sulfuric acid to react on litharge or similar lead compounds in the presence of a small amount of an acid or salt promoting the solution of the lead and with resulting production of a basic lead sulfate, and to cause such precipitated sulfate to react with a soluble chromate to convert the sulfate into chromate. The conversion of the sulfate into chromate, by such process, is not complete, and the product produced differs from the product produced by the present process. In the present process, where sulfuric acid is added first to the litharge, and lead sulfate produced, the sulfate is not subsequently converted into chromate, but the sulfate remains as such and the chromate is subsequently produced by further reaction upon the unreacted litharge. When the chromate together with its chemical equivalent of acid (for example sulfuric acid) is added at the proper rate to the litharge suspension in water, at no time during the process is either normal or basic lead sulfate present regardless of the sulfate ion present.

It has also been proposed to prepare lead chromate pigments by the gradual addition of chromic acid to a suspension of litharge or similar lead compound in water and with the addition of a small amount of catalytic acid or salt, such as acetic acid, which is relied upon to dissolve the lead compound to form e. g. lead acetate which then reacts with the chromic acid, with regeneration of acetic acid. In the present process, even where acetic acid is used, it is not regenerated, but is converted into sodium acetate. The presence of acid in excess, even temporarily, tends to precipitate lead sulfate or lead chromate directly Any solvent action which the anion of the acid employed has on the litharge will serve to speed up the reaction, but the reaction when carried out in the presence of sulfate ions results in the production of lead sulfate which remains as such, or in the production of sulfate-chromate products as the product of the process, depending upon the proportions of soluble chromate, acid, and. litharge employed.

The process of the present invention is particularly advantageous for the commercial manufacture of lead chromate and of lead chromatecontaining pigments in that it enables the cheapest raw materials to be employed, namely, litharge, sodium bichromate and sulfuric acid.

of the class consisting of chromates and bichroently of the sulfate content of the solution. This control depends on the excess of acid over bichromate in the reaction mixture and is capable of giving exact control of the sulfate content.

By the present process it is also possible to prepare a pure lead chromate in the presence of large quantities of ions such as sulfate, and without the formation of insoluble lead salts of such ions, such as lead sulfate, by regulation of the proportion of acid and bichromate employed. In

, no other process with which we are familiar is a pure lead chromate obtained in the presence of sulfate.

In carrying out the process of the present invention, either for the production of a pure lead chromate pigment, or for the production of a mixed chromate-sulfate pigment of accurately controlled proportions of chromate and sulfate, or for the production of other mixed chromatecontaining pigments, an alkaline condition is ,maintained throughout the process up to the point where all of the litharge or other insoluble basic lead compound employed has been used up to form either lead chromate alone, where that product is desired, or a mixture of lead chromate and lead sulphate of regulated proportions, or the production of other desired mixed pigments. The process is controlled not only by maintaining an alkaline condition throughout, but by controlling the proportions of acid and bichromate (or chromate) employed for reaction with the lead compound; and with gradual and regulated addition of the reagents so as to avoid an excess of acid and the precipitation of insoluble lead salts on the surfaces of the litharge particles.

The improved pigments produced by the present process are definitely more voluminous than any other product with which we are familiar. The product is characterized by good light fastness and maintains its clear tone exceptionally well on dilution with inert fillers; and it has other advantageous properties, such as those hereinbefore pointed out.

We claim:

1. The process of producing lead chromate pigments which comprises gradually adding, to a rapidly agitated suspension in water of a difii cultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, a mixture of an acid and a compound mates, in an amount and in proportions suflicient to convert the lead compound with substantial completeness into the desired pigment, the said mixture being added at a rate not exceeding the rate of solution of the lead compound so as to maintain an alkaline reaction in the suspension throughout the process, the amount of water in said suspension being sufiicient to permit substantially complete reaction of the difficultly soluble basic lead compound.

2. The process of producing a lead chromate pigment substantially free from sulfate which comprises gradually adding, to a rapidly agitated suspension inwater of a difiicultly soluble basic lead compound of thegroup consisting of lead oxides, carbonates and hydroxides, a bichromateacid mixturecontaining sulfate ions at a rate not exceeding thev rate of solution of the lead compound'so as to maintain an alkaline reaction in the suspension throughout the process, the amount and proportions of said bichromate-acid mixture being suflicient to convert the lead compound with substantial completeness into the desired pigment, the amount of water in said suspension being sufficient to permit substantially complete reaction of the diflicultly soluble basic lead compound.

3. The process of producing lead chromate pigments which comprises gradually adding, to a rapidly agitated suspension in water of a difficultly soluble basic lead compound ofthe group consisting of lead oxides, carbonates and hydroxides, a mixture of sodium bichromate and sulfuric acid in amount and in proportion sumcient to convert the lead compound with substantial completeness into the desired pigment, the said mixture being added at a rate not exceeding the rate of solution of the lead compound so as to maintain an alkaline reaction in the suspension throughout the process, the amount of water in said suspension being sufficient to permit substantially complete reaction of the difiicultly soluble basic lead compound.

4. The method of producing a leadchromate pigment substantially free from sulfate which comprises gradually adding, to a rapidly agitated suspension in water of a diflicultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, a mixture of sodium bichromate and sulfuric acid in stoichiometrical porportions and at a rate not exceeding the rate of solution of the lead compound so as to maintain an alkaline reaction in the suspensionthroughout the process, the amount of such bichromate-acid mixture being suflicient to convert the lead compound with substantial completeness into the desired lead chromate pigment, the amount of water in said suspension being sufficient to permit substantially complete reaction of the difiicultly soluble basic lead compound.

5. The method of producing lead chromate pigments containing lead chromate and lead sulfate which comprises gradually adding, to arapidly agitated suspension in water of a difiicultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, a mixture of sodium bichromate and sulfuric acid containing an excess of acid over that corresponding to the bichromate, said mixture being added at a rate not-exceeding the rate of solution of the lead compound so as to maintain an alkaline reac tion in the suspension throughout the process, the amount of said acid-chromate mixture being sufiicient to convert the lead compound with substantial completeness into the desired pigments, the amount of water in said suspension being sufficient to permit substantially complete reaction of the dirficultly soluble basic lead compound.

6. In the production of lead chromate pigments by the gradual addition, to a rapidly agitated suspension in water of a diflicultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, of a mixture of an acid and a compound of the class consisting of soluble chromates and bichromates, at a rate not exceeding the rate of solution of the lead compound so as to maintain an'alkaline reaction in the suspension during such addition, the amount of water in said suspension being sufficient to permit substantially complete reaction of thedifficultly soluble basic lead compound. the further improvement which comprises adding a limited and regulated amount of acid to the suspension before the addition of said chromateacid mixture, the amount of said acid-chromate mixture being suificient to convert the lead compound with substantial completeness into the desired pigments.

7. In the production of lead chromate pigments containing lead sulfate by the gradual addition, to a rapidly agitated suspension in water of a difficultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, of a bichromate-acid mixture containing sulfate ions, at a rate not exceeding the rate of solution of the lead compound so as to maintain an alkaline reaction in the suspension throughout the process, the further improvement which comprises gradually adding, in a similar manner, to the suspension, a limited and regulated amount of sulfuric acid, to convert part of the lead compound into a lead sulfate, before adding the bichromate-acid mixture to convert the remainder of the lead compound into pigment, the amount of water in said suspension being sufficient to permit substantially complete reaction of the difiicultly soluble basic lead compound.

8. The method of producing lead chromate pigment containing lead chromate and lead sulfate which comprises gradually adding to a rapidly agitated suspension in water of a diflicultly soluble basic lead compound of the group consisting of lead oxides, carbonates and hydroxides, a mixture of a compound of the class consisting of chromates and bichromates and an acid, containing an excess of acid over that amount equivalent to the chromate or bichromate, and sulfate ion not less than that equivalent to the desired lead sulfate, said mixture being added at a rate not exceeding the rate of solution of the lead compound so as to maintain an alkaline condition in the suspension throughout the process and the amount of lead sulfate in the pigment being limited and regulated by the excess of acid over chromate or bichromate in the reacting mixture, the amount of said acid-chromate mixture being sufficient to convert the lead compound with substantial completeness into the desired pigments, the amount of water in said suspension being sufficient to permit substantially complete reaction of the diflicultly soluble basic lead compound.

9. Improved lead chromate pigments produced in accordance with the process of claim 1, said pigments being characterized by extraordinary flocculence when dispersed in water, high tinting strength and high oil absorption.

10. Improved lead chromate pigments produced in accordance with the process of claim 2, said pigments being characterized by extraordinary flocculence when dispersed in water, high tinting strength and high oil absorption.

11. Improved lead chromate pigments produced in accordance with the process of claim 3, said pigments being characterized by extraordinary flocculence when dispersed in water, high tinting strength and high oil absorption.

12. Improved lead chromate pigments produced in accordance with the process of claim 4, said pigments being characterized by extraordinary flocculence when dispersed in water, high tinting strength and high oil absorption.

OMAR F. TARR. WINSLOW H. HARTFORD. 

